The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical circulators.
By directing signal flow in the proper direction, optical circulators can reduce system cost and complexity in optical equipment used in fiber optic networks. In complex optical networks, passive optical components are essential elements for sorting and delivering signals to their proper destination. To accomplish this control, the optical-signal flow though the sequential ports of a circulator is guided in a fashion comparable to that of vehicles entering and leaving a traffic circle. A circulator transmits an incoming signal entering Port 1 to Port 2 while transmitting another incoming signal from Port 2 to Port 3, and another from Port 3 to Port 4 etc. The number of ports can be increased arbitrarily, and it is possible to have fully circulating devices, in which light entering the last port exits the first port, and quasi-circulating devices wherein the light from the last port does not return to the first port, this quasi-circulator is the most common type. The performance advantages of optical circulators make them indispensable for routing bidirectional optical traffic. Firstly, optical circulators are low-loss devices, unlike splitters that incrementally add 3-dB losses for each device used. Secondly, optical circulators have high adjacent port isolation and eliminate the need for external isolators.
As fiber optic communication systems increase in complexity and functionality, the demand for increased capacity and efficient (low loss) signal routing management increases. For example, in duplex (bi-directional) transmission, the conventional use of fused fiber 3 dB couplers costs the system more than 6 dB in loss. The use of optical circulators in such cases saves about 5 dB""s due to the ability of circulators to route the signal in its entirety in the desired direction. Optical circulators are also important and enabling components in ADD/DROP applications. Optical circulators are forecast to play a significant role in duplex transmission, optical time domain reflectometry (OTDR) measurement systems, wavelength division/multiplexing (WDM) transmission systems and Erbium (Er) doped fiber amplifiers (EDFA).
Conventional fiber optic circulators are generally bulky and complex in design. This is caused by the fact that these designs offset the returning beam with respect to the forward beam, leading to the necessity of one collimating lens per port or fiber. FIG. 1 illustrates a conventional four port fiber optic circulator, it is seen that there are four collimating lenses (one for each of the optical fibers) and that, in this example, each fiber is disposed at a 90xc2x0 angle to each other. As is shown in FIG. 1 the light from port 1 is directed to port 2, the light from port 2 is directed to port 3, and the light from port 3 is directed to port 4. The use of prisms and reflectors is seen to result in a relatively bulky design. Furthermore the complexity of such designs means that alignment of the various fibers, lenses, reflectors and optical components is time consuming and thus contributes to the expense of the circulator. Examples of this type of circulator is found at U.S. Pat. No. 4,464,022 to Emkey; U.S. Pat. No. 5,212,586 to Van Delden and U.S. Pat. No. 5,818,981 to Pan et al.
The present invention is directed to an optical circulator of the inline type which provides a more compact circulator by displacing the beams in angle instead of position. This has the advantage of using one collimating lens per two or more fibers with all of the input and output fibers lying parallel to each other. The reduction in component count also greatly simplifies the necessary alignment of the components and thus reduces cost. FIGS. 2 illustrates schematically the principle of the inline circulator displacer. FIG. 2a shows transmission from optical fiber 1 to fiber 2 (this is called the forward direction). Light exiting fiber 1 is collimated by the input lens and passes through the circulator parallel to the input. This beam couples, through the output lens, with fiber 2. Light launched from fiber 2 will be collimated through the output lens (acting now as an input lens) and passes through the circulator displacer (this is called the reverse direction). Now the propagation direction through the displacer is opposed to that of the previous situation (from fiber 1 to 2). This leads to an angular displacement of the beam exiting the displacer with respect to the beam entering it. This displaced beam will now pass through the input lens and couple optimally with fiber 3 as shown in FIG. 2b. Launching from fiber 3, the circulator displacer will not deviate this beam as the propagation direction is the same as launching from fiber 1 but the beam enters at an angle. Now the light from fiber 3 will couple with fiber 4. In this case the separation of fiber 1 and 3 is the same as that of fibers 2 and 4 as shown in FIG. 2c. It is seen that fibers 1 and 2 are collinear, with fiber 3 being disposed parallel to and lower than fiber 1 and fiber 4 being disposed parallel to and above fiber 2. Additional fibers would be disposed in a similar manner i.e. odd numbered fibers lying parallel to and below that of fiber 1 and even number fibers lying parallel to and below that of fiber 2.
In the following discussion, all diagrams will describe inline circulator displacers. This assumes that the light is launched from the fiber and collimated through an input lens and further on imaged back into a fiber using an output lens, these components are not shown in the following drawings for the sake of clarity but should be assumed to be present.
A variety of approaches to providing inline optical circulators have been taken, in U.S. Pat. Nos. 5,909,310 and 5,930,039 to Li et al illustrate inline optical circulators that utilize two birefringent displacers, a birefringent wedge set (oppositely disposed wedges of birefringent material that have their optical axes oriented orthogonally to each other and to the direction of beam propagation) together with Faraday rotators, xc2xd wave plates and an birefringent beam path deflectorxe2x80x9d. U.S. Pat. No. 6,014,475 to Frisken describes an inline optical circulator that utilizes multiple birefringent displacers, a birefringent wedge (in one embodiment) together with Faraday rotators, xc2xd wave plates and one or more xe2x80x9cimagingxe2x80x9d means (a compound lens). The more components that are disposed in the beam path of a circulator, the more critical the alignment of each component with respect to the others becomes. Even very small misalignments can cause loss of efficiency or total failure of the circulator, the requirement for complex alignment procedures will increase the cost of the device and render certain designs impractical. The use of waveplates in the circulators increases the wavelength sensitivity, which can be problematic as WDM systems become more commonplace. The present invention is directed to providing optical circulators that reduce component count and hence cost, the present devices are designed so that alignment is straightforward and can be implemented without waveplates.
The present invention is directed to embodiments for an inline optical circulator utilizing birefringent displacers to split an incoming beam into two orthogonally polarized beams and thereafter to recombine the beams. The birefringent displacers are used in combination with non reciprocal devices (Faraday rotators), reflectors and waveplates and serve to route a signal serially to the next port. Specific embodiments utilize birefringent wedges which provide circulators with reduced component counts and simplified alignment of the components. Circulators which do not utilize waveplates and thus are relatively wavelength insensitive are also described.